We have recently isolated a new class of conditionally lethal mutations in Chinese hamster cells in culture: these cells are deficient in oxidative energy metabolism. One such mutant has been characterized to be defective in NADH-CoQ reductase, and work is in progress with two other mutants: one with a very low level of cytochrome oxidase, the other with a very much reduced activity of succinate dehydrogenase. Further biochemical and genetic analyses with these and other mutants are planned. A second major project in our lab is concerned with temperature-sensitive cell cycle mutants. One such mutant has been characterized to be also defective in glycoprotein biosynthesis and further work is planned. We are also trying to develop new methods for the isolation of such mutants. BIBLIOGRAPHIC REFERENCES: Energy metabolism in respiration-deficient and wild type Chinese hamster cells in culture. M. Donnelly and I. E. Scheffler, J. Cell. Physiol., in press (July 1976). A respiration-deficient Chinese hamster cell line with a defect in NADH-CoQ reductase. L. DeFrancesco, I. E. Scheffler and M. J. Bissell, J. Biol. Chem., in press (July or August, 1976).